"and will to the best of my ability, preserve, protect and defend the Constitution of the United States.
The President swears he'll defend the Constitution. It, in turn, guarantees & protects our rights. A union has to protect & defend the contract. Our work rules & pay are only guaranteed if we defend the contract. Allow one part to be breached & quickly, the guarantees become more a guideline. Or to put it in Options terms: "gray areas."
This is why a union fights battles that sometimes may appear as unpopular. Strictly speaking, they're only defending (as they're required to do) the very contract you voted in.
I can't comment on whether the contract is amendable during its term or whether an LOA (Letter of Agreement) serves legally as an amendment. Regardless: at any time, the company could write an LOA guaranteeing provisions for all union members. They could do it pretty fast too, as Uncle Kenn demonstrated last month with his Citation Air tomfoolery/chicanery/smoke screen.
The Constitution has been amended to ensure no one is discriminated against because of race, religion, etc. The contract also can not discriminate against any of its members, including those ignoramuses like Rupert who thinks one person is more equal than the others or Bluenose, who thinks the union is taking money out of its members pockets. This is not to say that SM isn't an outstanding pilot, a fair CA, or a good guy to be stuck with for 8 days. 'Cause he is. He deserves pay protection. But while we're at it, so do hundreds of other FO pilots.